There are several types of hollow-fiber membranes for use in treating water, such as microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes. These hollow-fiber membranes are widely used for treating drinking water, sewage water, drain water and the like. Examples of the materials for the hollow-fiber membranes include polyethylene, polypropylene, polyvinylidene fluoride, polysulfone, polyether sulfone, polyacrylonitrile, polyamide, cellulose, cellulose acetate, etc.
The following are examples of the basic properties required for hollow-fiber ultrafiltration membranes that are used for membrane separation.    (1) Excellent performance in removing target substances.    (2) High permeability relative to the substances that are to be passed through.
(Hereunder, the combined properties of (1) and (2) are referred to as fractionation characteristics.)    (3) High permeability relative to the fluid to be treated (permeability).
(Hereunder, the combined properties of (1), (2) and (3) are referred to as membrane separation characteristics.)    (4) Sufficient strength to resist breaking or leaking (strength).    (5) Fractionation characteristics that resist a decrease over time (retention of fractionation characteristics).    (6) Permeability relative to the fluid to be treated that resists a decrease over time (retention of permeability).
(Hereunder, the combined properties of (5) and (6) are referred to as retention of membrane separation characteristics.)
In addition to the above properties, separation membranes, other than disposable membranes, such as those used for artificial dialysis, are required to have the following properties.    (7) Excellent recovery of fractionation characteristics by cleaning (recovery of fractionation characteristics).    (8) Excellent recovery of permeability by cleaning (recovery of permeability).
(Hereunder, the combined properties of (7) and (8) are referred to as recovery of membrane separation characteristics.)
In the filtration operation of drinking water production, wherein tap water is produced by purifying fresh water, such as river water, lake water, ground water, etc., sand filtration or a combination of coagulation sedimentation and sand filtration has conventionally been employed as the filtration process. However, due to concerns about contaminating water sources with cryptosporidium, which is highly resistant to chlorine and may not be completely detoxified by sterilization using chlorine after filtration, there is currently an increasing tendency for membrane filtration techniques that more easily and reliably remove contaminants to be used either singly or in combination with other techniques, such as coagulation sedimentation and sand filtration. In the desalination of seawater using a reverse osmosis membrane as well, the seawater that is to be supplied to the reverse osmosis membrane is subjected to coagulation sedimentation, sand filtration or like pre-treatment (clarification process) to remove contaminants in advance, and then supplied to the reverse osmosis membrane to be desalinated. Here too, there is a tendency for contaminants to be removed by employing membrane filtration rather than coagulation sedimentation and sand filtration, or by combining membrane filtration with other techniques.
As time passes in a membrane filtration process, fouling substances adhere to and are deposited onto the membrane surface on the side to which raw water is supplied. This increases membrane filtration resistance and lowers the filtration efficiency. When this occurs, the fouling substances adhering to the membrane surface are removed by back washing, air scrubbing, feed water draining, chemical cleaning or a like cleaning operation to decrease the membrane filtration resistance. Thereafter, the membrane filtration is re-started. The membrane filtration process proceeds by alternately repeating the membrane filtration operation and cleaning operation. In the cleaning operation, electricity, water and other utilities are necessary, and water production is halted during the operation; therefore, it is preferable that the required cleaning operation frequency be as low as possible, and that the time necessary for the cleaning operation be as short as possible. This makes it highly advantageous to obtain a separation membrane that has little increase in membrane filtration resistance as time passes in the membrane filtration operation, and that greatly decreases the membrane filtration resistance by conducting a cleaning operation.
In prior art techniques, various techniques have been developed in order to suppress the increase in filtration resistance during the membrane filtration operation by hydrophilizing the membrane surface to enhance its anti-fouling property. One example of such a technique is the addition of polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA) or a like hydrophilic polymer to the stock solution that is used to produce a porous membrane formed of a hydrophobic polymer. This allows the hydrophilic polymers remaining in the formed membrane to increase the hydrophilicity of the membrane surface, thus enhancing its anti-fouling property. This is an excellent method in that membranes can be easily obtained with high productivity. However, a satisfactory anti-fouling property has not been attained.
In other prior art techniques, a membrane of a hydrophobic polymer is formed first, and the membrane is then subjected to various surface treatments to cover the surface of the hydrophobic polymer membrane with a hydrophilic polymer so as to enhance the anti-fouling property. These methods are more complicated than the method in which membranes are produced by adding a hydrophilic polymer to a membrane-production stock solution. Furthermore, the process control is difficult in these methods, resulting in numerous problems for practical use.
Both of the above-described types of prior art techniques are based on the technical concept of focusing on the chemical properties and chemical constituents of the membrane surface, and enhancing the anti-fouling property by hydrophilizing the membrane surface. Unlike the techniques described above, Patent Document 5 and Patent Document 6 consider the relationship between the shape of the membrane surface and the membrane separation characteristics. Patent Document 5 discloses an invention regarding a composite reverse osmotic membrane comprising a polyamide-based skin layer. Patent Document 5 also discloses that permeability can be improved by setting the specific surface area of the membrane surface on the side to which raw water is supplied to within a specific range. Patent Document 6 also discloses an invention regarding a composite reverse osmotic membrane comprising a polyamide-based skin layer. In the composite reverse osmotic membrane of Patent Document 6, when the average horizontal distance between the adjacent tops of the asperities on the surface to which the raw water is supplied is defined as X and the average difference between the adjacent tops and bottoms of the asperities is defined as Y, a high blocking performance can be obtained when X and Y satisfy a specific relationship. However, both Patent Document 5 and Patent Document 6 consider composite reverse osmotic membranes and do not teach any improvement in the anti-fouling property.
Patent Document 1: Japanese Examined Patent Publication No. 1995-090154
Patent Document 2: Japanese Patent No. 3594946
Patent Document 3: Japanese Patent No. 3216910
Patent Document 4: Japanese Unexamined Patent Publication No. 1999-179176
Patent Document 5: Japanese Unexamined Patent Publication No. 1997-19630
Patent Document 6: Japanese Unexamined Patent Publication No. 2005-169332